In the drive for higher integration and operating speeds in. LSI devices, the pattern rule is made drastically finer. The rapid advance toward finer pattern rules is grounded on the development of a projection lens with an increased NA, a resist material with improved performance, and exposure light of a shorter wavelength. In particular, the change-over from i-line (365 nm) to shorter wavelength KrF excimer laser (248 nm) brought about a significant innovation, enabling mass-scale production of 0.18 micron rule devices. To the demand for a resist material with a higher resolution and sensitivity, acid-catalyzed chemical amplification positive working resist materials are effective as disclosed in U.S. Pat. No. 4,491,628 and U.S. Pat. No. 5,310,619 (JP-B 2-27660 and JP-A 63-27829). They now become predominant resist materials especially adapted for deep UV lithography.
As is known in the art, the bilayer resist method is advantageous in forming a high-aspect ratio pattern on a stepped substrate. It is also known that in order that a bilayer resist film be developable with a common alkaline developer, high molecular weight silicone compounds having hydrophilic groups such as hydroxyl and carboxyl groups must be used.
Among silicone base chemically amplified positive resist compositions, recently proposed were those compositions for KrF excimer laser exposure comprising a base resin in the form of polyhydroxybenzylsilsesquioxane, which is a stable alkali-soluble silicone polymer, in which some phenolic hydroxyl groups are blocked with t-BOC groups, in combination with a photoacid generator (see JP-A 6-118651 and SPIE vol. 1925 (1993), p. 377). Positive resist compositions comprising as a base a silsesquioxane of the type in which cyclohexylcarboxylic acid is substituted with an acid labile group were proposed for ArF excimer laser exposure (see JP-A 10-324748, JP-A 11-302382, and SPIE vol. 3333 (1998), p. 62). A positive resist composition comprising as a base resin a silsesquioxane having hexafluoroisopropanol as a solubilizing group was proposed for F2 laser exposure (see JP-A 2002-55456). This polymer bears in its backbone a polysilsesquioxane containing a ladder skeleton obtained through polycondensation of a trialkoxysilane or trihalosilane. Also proposed as a resist base polymer having silicon pendants on side chains were silicon-containing (meth)acrylic ester polymers (see JP-A 9-110938, and J. Photopolymer Sci. and Technol., Vol. 9, No. 3 (1996), p. 435).
The silicon-containing polymers of (meth)acrylic ester type have the drawback that their resistance to dry etching with oxygen plasma is weak as compared with the silsesquioxane polymers. A low silicon content and a different polymer main skeleton account for this weak dry etching resistance. The siloxane pendant type (meth)acrylic ester polymer has the other drawback of poor developer wetting that it is likely to repel a liquid developer. SPIE vol. 3678 (1999), pp. 214, 241 and 562 describes a polymer of the trisilane or tetrasilane pendant type which is improved in alkali solubility by increasing a silicon content and endowing a silicon-containing group with an ability to be eliminated with acid. Although this polymer exhibits strong absorption at a wavelength of less than 200 nm on account of silicon-to-silicon bonds, it is fully highly transparent to radiation of 248 nm from a KrF excimer laser and used as a silicon-containing, acid-eliminatable group having improved etching resistance. Studies have been made on silicon-containing acid labile groups other than the foregoing (see SPIE vol. 3678 (1999), p. 420. The inventors proposed acid labile groups having silicon introduced therein (see JP-A 2001-278918 and JP-A 2001-158808). These polymers have the advantages of an improved acid elimination ability and prevention of T-top profiling and are characterized by a fully high transparency at the wavelength of an ArF excimer laser because carbon atoms intervene between silicon atoms to exclude silicon-to-silicon bonds.
We then presumed that the above-discussed problem can be solved by a resist composition comprising a polymer obtained by polymerizing a monomer having silicon attached to an unsaturated bond for polymerization, rather than the ester pendant.
For monomers having silicon attached to an unsaturated bond such as vinylsilane, neither radical polymerization of the monomer alone nor copolymerization of the monomer with norbornene or acrylic monomers is possible. They are copolymerized with hydroxystyrene by anionic polymerization. See J. Vac. Sci. Technol. B, 1986, 4, 422. Also silanes having a conjugated double bond are polymerized in the presence of living anion catalysts. See Macromolecules, 20, 2034-2035 (1987) and Macromolecules, 22, 1563-1567 (1989).
The copolymerization of allylsilane with maleimide is reported in Proc. SPIE, Vol. 1262, p. 110 (1990), and the copolymerization thereof with maleic anhydride reported in Proc. SPIE, Vol. 1466, P. 227 (1991). Allylsilane and maleimide form alternating copolymers. It is well known that vinylsilane and allylsilane copolymerize with sulfur dioxide as described in Proc. SPIE Vol. 1466, p 520 (1991). However, sulfur dioxide polymers are characterized by ease of cleavage upon exposure to light or plasma or electron beams. A phenomenon that lines were narrowed by electron beam irradiation during SEM observation was observed. It was also found that the sulfur dioxide polymers were less resistant to etching.
The inventors proposed in Proc. SPIE Vol. 5039 (2003), Proceeding No. 75, and JP-A 2002-348332, a polymer of a vinylsilane derivative, maleic anhydride, and a (meth)acrylic acid ester, having acid labile groups incorporated therein; in Japanese Patent Application No. 2002-192947, a polymer of a vinylsilane derivative, maleic anhydride, and norbornene carboxylate, having acid labile groups incorporated therein; and in Japanese Patent Application No. 2002-192866, a polymer of a vinylsilane derivative, maleic anhydride, and itaconic acid, having acid labile groups incorporated therein. The vinylsilane polymers, in which silicon atoms are directly bonded to the backbone, have a high Tg, high resistance to etching with oxygen gas, and high resistance to electron beam irradiation during SEM observation.
Bilayer resists having as a pendant a methacrylate having polyhedral oligometric silsesquioxane (POSS) as a pendant have been reported. The POSS has a cage-like structure. Its synthesis and cure properties thereof having an epoxy pendant are described in Macromolecules, Vol. 31, No. 15, p. 4970 (1998). The behavior of POSS-pendant methacrylic polymer when patterned with electron beams is described in Mater. Res. Soc. Symp., Vol. 636, D6.5.1 (2001).
JP-A 2001-305737 discloses that hexafluoropentyl alcohol-pendant POSS having acid labile groups substituted thereon is exposed to radiation of 157 nm. Proc. SPIE, Vol. 5039 (2003), Proceeding No. 49 shows the exposure properties at 157 nm of POSS pendant alpha-CF3 methacrylic polymer. It has been problematic that silicon methacrylic esters are prone to generation of silicon-containing outgases upon exposure at 157 nm, the silicon-containing material deposits on the surface of a projection lens to reduce its transmittance, and such deposits cannot be removed even by O2 cleaning. It was believed that only ladder polymers having a silsesquioxane skeleton in the backbone eliminate the generation of silicon-containing outgases. It was found that introduction of silsesquioxane into methacrylic polymers as a pendant eliminates the outgassing. The inventors also proposed an allyl POSS polymer in JP-A 2002-256033.